Lesions induced in DNA may be repaired by "error-proof" or "error-prone" repair mechanisms. Error-prone systems repair the lethal damage of the lesion, but in doing so may fail to restore the original nucleotide sequence, producing mutations. This error-prone process is called misrepair mutagenesis. Bacteriophage T4 has its own misrepair pathway, mediated by at least three genes, x, y and w. Mutations in two of these genes, x and y, suppress mutations of gene 49, a gene involved in DNA packaging. This allows the selection of alleles of x and y; ts alleles of x and y have been selected. These are temperature sensitive both for suppression of gene 49 amber mutants and for UV sensitivity. Amber alleles of x and y are being sought. Using these and other x and y alleles, the genes x and y will be mapped. The ts alleles will be used to study the time course of mutagenesis in T4.